1. Field of the Invention
The present invention relates to the field of oximeters for determining the oxygen saturation in the blood of a subject. More particularly, the invention is concerned with an oximeter using sigma-delta analog to digital converters.
2. Description of the Prior Art
Prior art oximeters detect the transillumination by red and infrared light of a body portion of an in vivo subject in order to determine the blood oxygen saturation of the subject. Analog detector signals representative of the transillumination are converted to digital signals and then processed to determine blood oxygen saturation. The digital signals need to present high resolution in order to provide sufficient accuracy. Typically, conventional analog to digital converters have been used such as sample and hold converters. A converter is needed for each red and infrared channel and for the calibration channel. The use of three converters represents substantial cost and adds to the size of the circuitry.
Other prior art oximeters have used an off-the-shelf converter chip for each channel that includes a sigma-delta modulator and digital filtering on the same chip. This still requires the use of three converters and does not allow flexibility in selecting digital signal resolution.